Acclimatization of Ficus benjamina: A Review

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University of Florida
Central Florida Research and Education Center - Apopka
CFREC-A Research Report RH-91-5

K. Steinkamp*, C.A. Conover and R.T. Poole**

Introduction

The most widely utilized tropical tree in the interiorscape industry is Ficus benjamina or weeping fig, a member of the family Moraceae and indigenous to southeast Asia. Ficus benjamina and its cultivars have been the focus of much of the foliage plant acclimatization research for the past 20 years because of their enormous popularity with the plant buying public and also because weeping figs are among the sun-shade plants most clearly benefiting from the acclimatization process.

Properly acclimatized weeping figs can make the transition from production to the interior environment when given proper fertilization and irrigation corresponding to available light levels. Non-acclimatized plants defoliate, and may die if leaf drop is severe. The extent of plant quality lost is dependent on a host of factors encountered during production, shipping and storage.

Light Effects

Early Ficus benjamina acclimatization studies were aimed at converting trees grown in full sun to shade plants able to adjust to low light interiors. Conklin (10) developed a system of "pre-acclimatizing" plants being grown for interior use when he found that plants placed in heavily shaded greenhouses and watered sparingly for 2 months prior to placement in interior settings consistently scored higher quality grades than plants not receiving the shade treatment. In 1973 Conover and Poole (11) determined that lowering light levels from a maximum of 12,000 ft-c utilized during production to 2,500 ft-c for just 12 weeks lessened leaf abscision on weeping figs subsequently placed in simulated interior environments having a light intensity of 50 ft-c 8 hours per day for ten weeks. Two years later, Conover and Poole (12) showed that containerized Ficus benjamina received higher plant grades, and dropped fewer leaves after 10 weeks in an interior setting if they were first acclimatized for at least 5 weeks under 1,250 to 2,500 ft-c (60% to 80% shade). Longer periods of shading supplied better quality plants. Plants also retained more leaves as interior light supplied 12 hours per day increased from 250 to 750 to 1250 ft-c. Light compensation point (LCP) is that point at which carbohydrates required by plants in respiration are equal to carbohydrates produced by photosynthesis. Photosynthetic rate must equal respiration rate if the plant is to survive. Plants with lower LCPs adapt better to low light interior environments. Joiner, Conover and Poole (25) found that Ficus benjamina LCP can be controlled by light levels utilized during production. High quality acclimatized plants were grown under shade in less time than the previously accepted acclimatization method of growing plants under full sun before subjecting them to an acclimatization period under low light. Conover and Poole (13, 14) reported that tree height, quality and foliage color increased when production shade levels were increased. Only trees grown under 80% shade continued to grow after 6 months in a simulated interior setting. Trees grown under shade, however, did not develop thick trunks necessary to support large specimen trees.

Turner, Reed and Morgan (40) compared two acclimatization methods at 5 light intensities to compare effectiveness in reducing leaf drop after placement indoors. The two methods tested were 1) production under 0, 20, 40, 60 and 80% shade levels prior to placement indoors, and 2) production in full sun followed by an 8 week acclimatization period under 20- 80% post-production shade levels, then placement indoors. Plants grown in full sun dropped the most leaves indoors. As production or post-production shade level increased, defoliation decreased. Plants grown under the 80% shade treatment dropped the lowest number of leaves.

Ficus benjamina grown in full sun develop thicker trunk diameters than shade grown trees. Conover and Poole (16) found wind movement in shaded production areas produced some thickening of trunks but not enough to support large specimen trees. Eight weeks of full sun followed by 16 weeks of 63% shade produced plants with thicker trunks but otherwise of the same quality as those grown in shade for the entire 24 weeks. Most Ficus benjamina grown today are acclimatized during production under shade, the exception being large specimen trees requiring thick trunks.

Although these experiments proved that production and post-production acclimatization procedures result in greater leaf retention and increased plant longevity indoors, research explaining the physiological changes taking place in leaves in response to environmental changes were needed to help develop optimum production regimes.

Leaf Structure

Many researchers have examined LCPs of shade and sun grown plants and concluded shade grown plants had lower LCPs than sun grown plants. New changes in leaf structure in response to production light levels were examined to help explain the shift in LCP. Peterson et al. (30) found that trees grown under high light intensities had smaller thicker leaves with two distinct palisade layers, while shade grown leaves had only one palisade layer. Fails, Lewis and Barden (18, 19, 20) studied the anatomy and morphology of sun and shade grown foliage and confirmed these findings. They also reported greater stomatal density in sun grown leaves, although shade grown leaves had more stomata per leaf. Sun grown leaves were small and thick with 2 layers of elongated palisade mesophyll cells and chloroplasts were aligned along the radial

cell walls. Shade grown leaves were larger, thinner and darker green with a single layer of short palisade cells. Chloroplasts were dispersed throughout the palisade cells and appeared to be larger than in sun grown plants. When net photosynthesis of plants grown in full sun and 50% sun was compared under various photosynthetically active radiation (PAR), shade grown leaves had a photosynthetic advantage over sun grown leaves at PAR comparable to lighting found in interiors. Sun grown leaves also transpired more at all PAR levels tested.

Light and Fertility

Light and fertilizer significantly affected LCP in Ficus benjamina in a 3 x 4 factorial experiment that tested plants grown under 0, 30, 55 and 80% shade fertilized with 700, 1400 or 2100 lb N+K/A/yr (6). Increasing shade levels decreased LCP at each shade level tested. Increasing fertilizer rates increased LCP, although fertilizer was less effective than shade level in altering LCP. Plants grown in full sun with 700 lb N+K/A/yr had LCPs nearly 3 times higher than the plants grown under 80% shade and 700 lb N+K/A/yr.

Research by Ceulemans, Gabriels and Impens (5) has shown that fertilization level during production influences plant morphology and structure. Johnson et al. (22) discovered that Ficus benjamina receiving higher nitrogen (N) rates had less leaves in the upper half of sun or shade grown plants . Increased N levels increased LCP of sun grown plants but reduced LCP of plants grown under 47% light exclusion. Higher N fertilization increased leaf development in lower and mid plant portions of the tree which increased LCP and potential leaf drop on plants grown in full sun. Shade grown plants had reduced LCPs, lower carbohydrate levels in leaves and roots, more leaf chlorophyll, were of higher quality and had longer post-harvest life than sun grown plants. Milks et al. (28) found sun grown plants had twenty-seven percent more leaf carbohydrate than those grown under 65% light exclusion (4180 ft-c), so reduction of LCP with increased N could be an interaction of N and low reserve carbohydrate level. Increased potassium (K) allowed more carbohydrate translocation to the root system.

Joiner, Johnson and Krantz (27) in testing 2 light levels, 3 N and 3 K levels on LCP, shoot and root growth, canopy distribution and leaf tissue nutrient content of Ficus benjamina found that plants grown under 47 % shade for 7 months had significantly lower LPCs than plants produced under full sun. N level slightly affected compensation point but K level had no effect. Higher N levels increased shoot growth and shade grown plants receiving high N levels had a higher shoot/root ratio. Light level did not affect amount of carbohydrates in the leaves but sun grown plants had more root carbohydrates. Root carbohydrate levels also increased when K levels were increased.

Johnson et al. (24) found that leaf diffusive resistances of Ficus benjamina grown in full sun were lower than those from 47% light exclusion partially explaining why sun grown plants need more water than shade grown plants. Fifty-three percent more stomata were found in sun leaves. High KC1 increased transpiration rates, and foliar levels of K were higher in sun grown plants but unaffected in shade plants. Acclimatization also affected respiration rate and as a plant became fully acclimatized respiration rate declined dramatically which reduced carbohydrate requirements. Light intensity and KC1 affects on transpiration could modify water requirements and practices in growing Ficus benjamina.

Shipping

Plants can lose leaves during shipping and storage due to water stress because of limited growing medium volume. Irrigation frequency during production affected leaf drop when acclimatized plants were placed indoors. Johnson, Ingram and Barrett (21) reported that plants watered at 3 day intervals during production dropped fewer leaves when moved to simulated interior environments than plants watered at 6 and 9 day intervals. However, results of research by Peterson, Sacculus and Darken (31, 32), while showing that water stress is a contributing factor in leaf abscission, suggested that "preconditioning" by exposing plants to water stress would not be economically feasible because growth rates of Ficus were dramatically reduced by water stress during production and leaf abscission was not appreciably reduced when plants were later subjected to severe water deficits. Ficus benjamina could not be "pre-conditioned" to lessen leaf drop in times of severe water stress.

Ficus benjamina sometimes experience substantial leaf drop during shipping and dark storage. Researchers in the late 70's and early 80's turned their attention to procedures to determine the effects of shipping and storage environments on acclimatized weeping figs. Poole and Conover (36, 37) found that quality of Ficus benjamina grown under 63% shade was much better than quality of plants grown under full sun or 30% shade when test plants were placed in interior environments for 12 weeks following a period of 0, 5, 10, or 15 days of storage in complete darkness at 60-65F and 655% relative humidity. The plants receiving higher fertilizer and light levels during production had more leaf abscission during simulated shipping and later in the interior environment. Longer storage time also increased amount of leaf drop.

Researchers found that storage duration and temperature also affected the quality of Ficus benjamina. Collins and Blessington (8) dark-stored Ficus benjamina for 4, 8, or 12 days at 37, 45, 70, 95 or 102F then placed plants in interiors for 30 days. Plants were not damaged when stored at 70F or 95F or when stored for 4 days. Leaf loss and foliar damage were more severe as dark storage time increased from 4 to 12 days. After subjecting large Ficus to storage periods of various temperatures, Poole and Conover (38) found that specimen trees having over 300 leaves and held at 50F had the highest plant grade (4.2 based on a quality scale of 1 = poor, unsalable, 3 = fair, salable, 5 = excellent quality) 12 weeks after removal from storage. Plants shipped at 66F, the temperature closest to the most commonly used shipping temperature, received a lower plant grade of 3.0. Buck and Blessington (3, 4) held Ficus at 40, 70 or 98F for 3, 6 or 9 days. Plants were adversely affected during simulated shipping by 40 and 98F, with leaf loss increasing with exposure time. Foliar damage was severe with plant grade the lowest for weeping figs exposed to 98F for 6 or 9 days. Plants held at 70F showed no foliar damage and only slight loss in quality regardless of shipping duration. After 8 weeks in a simulated interior environment plants held at 40 and 98F during simulated shipping had not recovered lost quality and showed even more severe foliar damage and greater leaf loss. Chlorophyll content of leaves decreased as storage temperatures increased.

Leaf Shine

Some foliage plant producers spray plants with foliage shine materials prior to shipment. Joiner, Conover and Poole (26) found that leaf shine compounds applied to Ficus benjamina prior to storage raised the LCP reducing tolerance to low light stress. Treated plants lost about 3 times more leaves compared to untreated plants within 2 weeks of leaf shine application indicating treated plants need higher interior light levels to maintain the same quality as untreated plants.

Foliage plants could be subjected to water stress during long-term transit, which can contribute to leaf abscission. Peterson and Blessington (34) applied Wilt-Pruf (Nursery Specialty Co., Greenwich CT) to Ficus benjamina foliage prior to storage to evaluate the influence of an antitranspirant on plant quality during dark storage. Wilt-Pruf increased leaf drop after dark storage treatments and plants were not salable two weeks after being dark stored for 8 and 12 days. Chlorophyll content of leaves decreased after 4, 8 and 12 days of storage.

Growth Regulators

(=Alpha)
Foliage plants often develop etiolated or spindly new growth when subjected to extended periods of low light levels such as those encountered during shipping, dark storage, display in retail shops and in dimly lit interiors. Peterson and Blessington (35) treated Ficus benjamina with the growth regulator ancymidol [-cyclopropyl--(p-methoxyphenyl)-5-pyrimidinemethanol] to evaluate its potential for controlling undesirable postharvest internode elongation and increasing Ficus postharvest keeping quality during dark storage and under incandescent (INC) and Cool White fluorescent (CWF) lights indoors. Plants were treated with ancymidol as a soil drench using 1.0 mg/15 cm (6 inch) pot using a standard volume of 200 ml (7 oz.)/pot two months after potting. All ancymidol treated plants also received a 200 ppm spray two months after the soil drench treatment. Plants were dark stored for 4, 8 and 12 days in shipping boxes before being held 4 months in an interior environment under 110 ft-c. Ancymidol treated plants dropped fewer leaves compared to untreated plants during all dark storage times tested. Treated plants also received higher plant grades than untreated plants for all dark storage times tested after plants were held under both lamp sources in interior environments for 4 months. Ficus under INC lamps lost the fewest number of leaves and scored the highest quality grades.

Johnson, McConnell and Joiner (23) treated weeping figs to the growth regulator ethephon at rates of 500 and 1000 mg (active ingredient) /25 cm (10 inch) pot applied as a soil drench, to determine effects of ethephon on growth responses, carbohydrate content, anatomical changes and leaf drop of plants grown under full sun and 47% sun exclusion. Treated sun and shade grown plants developed a prostrate growth habit and set fruit. Leaves of sun plants had a mesophyll with multiple palisade layers, while shade plants had only limited regions of multiple cells. Ethephon treatments reduced intercellular spaces in palisade and spongy mesophyll cells, especially near leaf margins. High shoot/root ratios, reduced leaf area and heavy leaf drop during 3 months in an interior environment occurred with ethephon treatment, with plants grown in full sun affected more than shade grown plants.

(=Alpha)
Barrett and Nell (1) reported no increase in leaf drop after limited observation of plants placed in interior environments after treatment with the growth retardants ancymidol [- cyclopropyl-- (p-methoxyphenyl)-5-pyrimidinemethanol], paclobutrazol [1-(4-chlorophenyl)-4, 4-dimethyl-2-(1,2,4-triazol-1-yl)] and EL-500 [ -(1-methylethyl)--(4-(trifluoromethoxy) phenyl)-5-prymidinemethanol], although these experiments primarily focused on the three growth regulators abilities to limit internodal length during production. More research examining various growth regulators ability to control growth of Ficus benjamina in interior environments is needed.

Interior Studies

In recent years, interiorscapers have become concerned about the possible side effects of continuous 24 hour lighting on their plantings in airports, shopping malls and other areas. Questions have been raised about the effects different kinds of artificial light sources have on plants and which if any is superior. Collins and Blessington (9) grew Ficus benjamina under 3000 or 6000 ft-c for 5 months then held plants indoors for 12 weeks under INC or CWF lights at light intensities of 75 or 150 ft-c PAR for 12 hours a day. Chlorophyll content was greater in plants grown under 3000 ft-c and plants dropped fewer leaves when held under 150 ft-c.

Collins and Blessington (7) also examined the effects of dark storage, light source and light duration in an interior environment on the postharvest keeping quality of Ficus benjamina. Ficus benjamina was stored in the dark for 0, 3, 6, 9 or 12 days then held in an interior environment for 12 weeks under 6, 12 or 24 hours of light per day from either INC or CWF lamps at 149 ft-c PAR. Plants stored for shorter periods lost fewer leaves and received higher plant quality grades. Plants lighted for 24 hours per day had less leaf drop and better plant grade than those from shorter light periods. Chlorophyll content of leaves increased as light duration increased for plants held under the CWF lamps. Plants lighted for 6 hours per day under INC lamps had the lowest chlorophyll content after 12 weeks indoors.

Conover, Poole and Nell (17) grew Ficus benjamina for 1 year under 100 or 200 ft-c from CWF lamps for 12, 18 or 24 hours per day. Plants produced with the continuous 24 hour lighting had the lowest quality scores and more chlorosis than plants exposed to 12 to 18 hour days. After one year the best plants were produced under 200 ft-c for 12 or 18 hours per day. These two experiments suggest that continuous 24 hour lighting after dark storage or shipping might be beneficial for a limited amount of time but becomes detrimental after extended periods.

Turner, Morgan and Reed (41) grew Ficus benjamina under the following light regimes for 1 year 1) 100% PAR from fluorescent, 2) 70% PAR from fluorescent plus 30% from incandescent, and 3) 50% each from Gro-Lux and Gro-Lux Wide Spectrum fluorescent. All light intensities were standardized at a total of 150 ft-c for 4 months then 100 ft-c for 8 months. No light source was proved superior for maintenance of the plants for 1 year indoors. In a second experiment three fertilizers were tested on plants receiving 150 ft-c for 3 months then 90 ft-c for 9 months. Fertilizers tested were: 1) soluble fertilizer (Peter's 20-20-20, Peters Fertilizer Products, P.O. 789, Fogelsville, PA 18051) added to the irrigation water weekly at 200 ppm N:88 ppm P:166 ppm K; 2) slow-release fertilizer (Osmocote 14-14-14, 3 month release, Grace-Sierra Co., Milpitas, CA 95035) applied as a top dress every three months at 4.1 g/6 inch pot (0.57 g N:0.25 g P:0.47 g K); and 3) an unfertilized control. At the end of one year the effects of fertilizer treatment were found to be minimal. These results agree with research by Conover and Poole (15) who also found few differences between slow-release and liquid fertilization at low light intensities during a 12 month study. Conover et al.(17) found an increase in chlorophyll content of leaves with an increase in slow-release fertilizer rate, but only a limited increase in plant quality after one year in the interior.

Cultivars

In the past few years some research has focused on genotype selection as a means to minimize leaf drop and enhance postharvest keeping quality of Ficus benjamina. Scientists wanted to know how much of the physiological, anatomical and morphological traits of Ficus induced by environmental factors were determined genetically. Steinitz, Ben-Jaacov and Hagiladi (39) determined that cultivar differences in Ficus benjamina were a major factor affecting leaf drop during shipping, dark storage and subsequent performance in interior environments. In another experiment Ben-Jaacov, Ziv and Steinitz (2) grew three clones of Ficus benjamina with similar morphology under 3 light intensities and variations in the morphological development of the clones in response to light intensity were recorded. Plants were dark stored for 2 weeks, then placed in an interior environment or placed directly into the interior setting without undergoing the dark storage phase. The length of dark storage promoted leaf abscission and the response pattern to light intensity during production, with regard to leaf drop during simulated interior conditions, showed a clear clone-dependent specificity.

Ottosen and Hoyer (29) reported up to 42 % difference in leaf abscission of some selected fast growing clones of Ficus benjamina after simulated shipping of pot plants in darkness followed by low light conditions in a simulated interior environment. The clones that grew faster under low interior light also had superior keeping quality indoors. This research suggests the possibility of selecting genotypes capable of fast growth and good keeping quality under interior conditions through plant breeding programs.


*Technical assistant.
**Professor and Center Director (retired 7/96), and Professor of Plant Physiology respectively, Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703-8504.


Literature Cited

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